net: allow better page reuse in splice(sock -> pipe)

splice() from socket to pipe needs linear_to_page() helper to transfert
skb header to part of page.

We can reset the offset in the current sk->sk_sndmsg_page if we are the
last user of the page.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -1565,6 +1565,9 @@ static inline struct page *linear_to_page(struct page *page, unsigned int *len,
 	} else {
 		unsigned int mlen;
 
+		/* If we are the only user of the page, we can reset offset */
+		if (page_count(p) == 1)
+			sk->sk_sndmsg_off = 0;
 		off = sk->sk_sndmsg_off;
 		mlen = PAGE_SIZE - off;
 		if (mlen < 64 && mlen < *len) {